The unique nature of bacteria, which can thrive even under extreme conditions, poses a major operational challenge in membrane processes.1,2,3 Bacteria tend to form a cohesive biofilm on membrane surfaces4, leading to an increase in the hydraulic resistance to permeation flow5. In desalination membranes, biofilms also intensify concentration polarization effects, leading to biofilm enhanced osmotic pressure and subsequent decline in product water flux and salt rejection6. Despite many past studies on biofouling formation mechanisms and control strategies7,8, a solution for preventing biofilm formation in membrane systems is still elusive9. Therefore, it is of paramount importance to develop improved biofouling control strategies for more sustainable operation of membrane systems for desalination and wastewater reuse. 1 Baker, J. S., Dudley, L. Y., 1998. Biofouling in membrane systems—a review. Desalination 118 (1-3), 81-89.2 Saeed, M. O., Jamaluddin, A. T., Tisan, I. A., Lawrence, D. A., Al-Amri, M. M., Chida, K., 2000. Biofouling in a seawater reverse osmosis plant on the red sea coast, saudi arabia. Desalination 128 (2), 177-190.3 Schneider, R. P., Ferreira, L. M., Binder, P., Bejarano, E. M., Goes, K. P., Slongo, E., Machado, C. R., Rosa, G. M. Z., 2005. Dynamics of organic carbon and of bacterial populations in a conventional pretreatment train of a reverse osmosis unit experiencing severe biofouling. Journal of Membrane Science 266 (1-2), 18-29.4 Flemming, H. C., Wingender, J., 2010. The biofilm matrix. Nature Reviews Microbiology 8 (9), 623-633.5 Herzberg, M., Kang, S., Elimelech, M., 2009. Role of extracellular polymeric substances (eps) in biofouling of reverse osmosis membranes. Environmental Science & Technology 43 (12), 4393-4398.6 Herzberg, M., Elimelech, M., 2007. Biofouling of reverse osmosis membranes: Role of biofilm-enhanced osmotic pressure. Journal of Membrane Science 295 (1-2), 11-20.7 Mansouri, J., Harrisson, S., Chen, V., 2010. Strategies for controlling biofouling in membrane filtration systems: Challenges and opportunities. Journal of Materials Chemistry 20 (22), 4567-4586.8 Matin, A., Khan, Z., Zaidi, S. M. J., Boyce, M. C., 2011. Biofouling in reverse osmosis membranes for seawater desalination: Phenomena and prevention. Desalination 281, 1-16.9 Vrouwenvelder, J. S., Kruithof, J. C., Van Loosdrecht, M. C. M., 2010. Integrated approach for biofouling control. Water Science and Technology 62 (11), 2477-2490.
For reverse osmosis (RO) membrane systems, feed waters with high biological activity require extensive pretreatment to lower the potential of biofouling development inside the RO membrane module. Specifically, both physicochemical removal methods and disinfection/oxidation of feed water to inactivate bacteria are commonly applied10,11. Physicochemical pretreatment processes significantly reduce the number of bacteria arriving at the RO membrane modules downstream, but they are not capable of removing all bacteria from the feed12,13,14,15,16. On the other hand, unlike low pressure membranes (microfiltration and ultrafiltration), thin-film composite polyamide membranes, the “gold standard” membranes for RO, degrade upon exposure to disinfectants and oxidants, such as chlorine17,18. This necessitates dechlorination of the feed water prior to contacting the RO membrane modules19,20. Other disinfection methods, such as UV, will not have residual activity within the membrane module21. Therefore, in the absence of biocidal activity inside the membrane module, bacteria that survive the pretreatment can proliferate and form a biofilm on the membrane surface during the desalination process22,23. 10 Lee, J., Oh, B. S., Kim, S., Kim, S. J., Hong, S. K., Kim, I. S., 2010. Fate of bacillus sp and pseudomonas sp isolated from seawater during chlorination and microfiltration as pretreatments of a desalination plant. Journal of Membrane Science 349 (1-2), 208-216.11 Prihasto, N., Liu, Q. F., Kim, S. H., 2009. Pre-treatment strategies for seawater desalination by reverse osmosis system. Desalination 249 (1), 308-316.12 Ghayeni, S. B. S., Beatson, P. J., Fane, A. J., Schneider, R. P., 1999. Bacterial passage through microfiltration membranes in wastewater applications. Journal of Membrane Science 153 (1), 71-82.13 Glueckstern, P., Priel, M., Gelman, E., Perlov, N., 2008. Wastewater desalination in israel. Desalination 222 (1-3), 151-164.14 Herzberg, M., Berry, D., Raskin, L., 2010. Impact of microfiltration treatment of secondary wastewater effluent on biofouling of reverse osmosis membranes. Water Research 44 (1), 167-176.15 Schneider, R. P., Ferreira, L. M., Binder, P., Bejarano, E. M., Goes, K. P., Slongo, E., Machado, C. R., Rosa, G. M. Z., 2005. Dynamics of organic carbon and of bacterial populations in a conventional pretreatment train of a reverse osmosis unit experiencing severe biofouling. Journal of Membrane Science 266 (1-2), 18-29.16 Voutchkov, N., 2010. Considerations for selection of seawater filtration pretreatment system. Desalination 261 (3), 354-364.17 Glater, J., Hong, S. K., Elimelech, M., 1994. The search for a chlorine-resistant reverse-osmosis membrane. Desalination 95 (3), 325-345.20 Shemer, H., Semiat, R., 2011. Impact of halogen based disinfectants in seawater on polyamide ro membranes. Desalination 273 (1), 179-183.19 Prihasto, N., Liu, Q. F., Kim, S. H., 2009. Pre-treatment strategies for seawater desalination by reverse osmosis system. Desalination 249 (1), 308-316.20 Schneider, R. P., Ferreira, L. M., Binder, P., Bejarano, E. M., Goes, K. P., Slongo, E., Machado, C. R., Rosa, G. M. Z., 2005. Dynamics of organic carbon and of bacterial populations in a conventional pretreatment train of a reverse osmosis unit experiencing severe biofouling. Journal of Membrane Science 266 (1-2), 18-29.21 Kim, D., Jung, S., Sohn, J., Kim, H., Lee, S., 2009. Biocide application for controlling biofouling of swro membranes—an overview. Desalination 238 (1-3), 43-52.22 Flemming, H. C., Schaule, G., Griebe, T., Schmitt, J., Tamachkiarowa, A., 1997. Biofouling—the achilles heel of membrane processes. Desalination 113 (2-3), 215-225.23 Saeed, M. O., Jamaluddin, A. T., Tisan, I. A., Lawrence, D. A., Al-Amri, M. M., Chida, K., 2000. Biofouling in a seawater reverse osmosis plant on the red sea coast, saudi arabia. Desalination 128 (2), 177-190.
Bacteria in biofilm are much more resistant to cleaning and biocides compared to planktonic bacteria24, underscoring the importance of preventing biofilm formation on the membrane. For mitigating biofilm formation, RO membrane manufacturers recommend intensive chemical cleaning every few months when the operational water flux decline to below 10% of the initial flux or when the required applied hydraulic pressure to maintain a constant product water flux increases by 15%25,26. Also non-oxidizing biocides, such as 2,2-dibromo-3-nitrilopropionamide (DBNPA), can be added to the feed during operation to inactivate bacteria within the module27,28. However, efficient use of these biocides requires high concentrations and relatively long exposure time (1-4 h), which increases cost and produces large waste streams. Therefore, there is a critical need to develop biofouling control strategies that lower biofouling potential inside the RO module by continuous inactivation of bacteria and by suppression of biofilm formation during filtration29. 24 Davies, D., 2003. Understanding biofilm resistance to antibacterial agents. Nature Reviews Drug Discovery 2 (2), 114-122.25 Hydranautics, 2011. Foulants and cleaning procedures for composite polyamide ro membrane elements. Technical Service Bulletin, Web site: http://www.membranes.com/docs/tsb/tsb107.pdf.26 Vrouwenvelder, J. S., Kruithof, J. C., Van Loosdrecht, M. C. M., 2010. Integrated approach for biofouling control. Water Science and Technology 62 (11), 2477-2490.27 Dow, F. M., Water chemistry and pretreatment: Biofouling prevention of filmtec elements with dbnpa. Tech Manual Excerpt, Web site: http://msdssearch.dow.com/PublishedLiteratureDOWCOM/dh_015 a/0901b8038015acc8038014.pdf?filepath=liquidseps/pdfs/noreg/8038609-8002036.pdf&fromPage=GetDoc.28 Hydranautics, 2013. Biocides for disinfection and storage of hydranautics membrane elements. Technical Service Bulletin, Web site: http://www.membranes.com/docs/tsb/TSB110.pdf.29 Saeed, M. O., Jamaluddin, A. T., Tisan, I. A., Lawrence, D. A., Al-Amri, M. M., Chida, K., 2000. Biofouling in a seawater reverse osmosis plant on the red sea coast, saudi arabia. Desalination 128 (2), 177-190.
In recent years, several studies demonstrated the potential to use metal nanoparticles (Me-NPs), such as silver nanoparticles (Ag-NPs)30,31,32,33, copper nanoparticles (Cu-NPs)34,35, zinc nanoparticles (Zn-NPs)36, and selenium nanoparticles (Se-NPs)37,38 as biocides in membrane separation processes. These metals exhibits strong antibacterial activity against numerous types of bacteria39, but their usage as biocide in membrane processes is mainly limited by the relatively high cost40. Addition of Me-NPs directly into the feed will consume large amounts of metal and therefore will not be economical. Localized loading of small amount of the metals in the vicinity of the membrane surface, where biofilms develop, is more viable for protecting the membrane from biofouling. Therefore, while the potential of different Me-NPs to effectively mitigate biofouling is well established in the literature, the loading method of Me-NPs on the membrane is one of the main hurdles that must be overcome. 30 Dror-Ehre, A., Adin, A., Markovich, G., Mamane, H., 2010. Control of biofilm formation in water using molecularly capped silver nanoparticles. Water Research 44 (8), 2601-2609.31 Liu, Y. L., Rosenfield, E., Hu, M., Mi, B. X., 2013. Direct observation of bacterial deposition on and detachment from nanocomposite membranes embedded with silver nanoparticles. Water Research 47 (9), 2949-2958.32 Mauter, M. S., Wang, Y., Okemgbo, K. C., Osuji, C. O., Giannelis, E. P., Elimelech, M., 2011. Antifouling ultrafiltration membranes via post-fabrication grafting of biocidal nanomaterials. Acs Applied Materials & Interfaces 3 (8), 2861-2868.33 Zodrow, K., Brunet, L., Mahendra, S., Li, D., Zhang, A., Li, Q. L., Alvarez, P. J. J., 2009. Polysulfone ultrafiltration membranes impregnated with silver nanoparticles show improved biofouling resistance and virus removal. Water Research 43 (3), 715-723.34 Ben-Sasson, M., Zodrow, K. R., Gengeng Q., Kang, Y., Giannelis, E. P., Elimelech, M., 2013. Surface functionalization of thin-film composite membranes with copper nanoparticles for antibacterial surface properties. Environmental Science and Technology 48 (1), 384-393.35 Akar, N., Asar, B., Dizge N., Koyuncu, I., 2013. Investigation of characterization and biofouling properties of PES membrane containing selenium and copper nanoparticles. Journal of Membrane Science 437, 216-226.36 Ronen, A., Semiat, R., Dosoretz, C. G., 2013. Impact of ZnO embedded feed spacer on biofilm development in membrane systems. Water Research 47, 6628-6638.37 Akar, N., Asar, B., Dizge N., Koyuncu, I., 2013. Investigation of characterization and biofouling properties of PES membrane containing selenium and copper nanoparticles. Journal of Membrane Science 437, 216-226.38 Low, D., Hamood, A., Reid, T., Mosley, T., Tran, P., Song, L., Morse, A., 2011. Attachment of selenium to a reverse osmosis membrane to inhibit biofilm formation of S. aureus. Journal of Membrane Science 378, 171-178.39 Harrison, J. J., Ceri, H., Stremick, C. A., Turner, R. J., 2004. Biofilm susceptibility to metal toxicity. Environmental Microbiology 6 (12), 1220-1227.40 USGS, 2012. Metal prices in the united states through 2010. Scientific Investigations Report 2012-5188, Web site: http://pubs.usgs.gov/sir/2012/5188/sir2012-5188.pdf.
When considering Me-NP loading procedure on the membrane, the aqueous dissolution of Me-NPs over time41,42,43, which can be exacerbated by the routine chemical cleaning, must be considered. Therefore, a suggested loading method of Me-NPs on the membrane must demonstrate also the ability to repeatedly recharge the Me-NPs on the membrane. This recharging procedure must be done on site, without disassembling the membrane module. Also, the Me-NP loading method must be economical in terms of chemical usage and time. 41 Kent, R. D., Vikesland, P. J., 2012. Controlled evaluation of silver nanoparticle dissolution using atomic force microscopy. Environmental Science & Technology 46 (13), 6977-6984.42 Kittler, S., Greulich, C., Diendorf, J., Koller, M., Epple, M., 2010. Toxicity of silver nanoparticles increases during storage because of slow dissolution under release of silver ions. Chemistry of Materials 22 (16), 4548-4554.43 Liu, J. Y., Sonshine, D. A., Shervani, S., Hurt, R. H., 2010. Controlled release of biologically active silver from nanosilver surfaces. Acs Nano 4 (11), 6903-6913.
Embedding Me-NPs or metal salts such as silver, copper, zinc, and selenium, in the RO polyamide layer during membrane fabrication44,45 will have low efficiency since only a small fraction of the Me-NPs will be exposed on the membrane surface. Furthermore, Me-NPs may only be embedded inside the membrane during membrane fabrication, and there is no opportunity for Me-NP recharge after dissolution and depletion. Binding of previously prepared Me-NPs on the membrane surface46 is a more plausible approach; however, this method will involve Me-NP synthesis and the use of a capping chemical agent, which increases cost. In addition, only a fraction of the synthesized nanoparticles in solution will eventually bind to the membrane surface. The principle suggested by Yang et al., of simultaneous fabrication and loading of Ag-NPs on the membrane by chemical reduction (in situ formation), while appearing to be a more favorable route for Ag-NP loading method on TFC-RO membrane47, is in fact not desirable because the Ag-NPs are bound to a spacer in Yang et al. and are not bound to the membrane. Furthermore, the use of several chemicals at relatively high concentrations (10 vol % ammonium-hydroxide, 0.4 M formaldehyde, 0.02 M silver nitrate), the use of ethanol as the reducing solution, and the relatively long reaction time (more than 1 h) of Yang et al. is also not desirable. For other metals such as copper, zinc, and selenium, an efficient procedure of simultaneous formation and binding of these Me-NPs on the membrane was not demonstrated yet. 44 Lee, S. Y., Kim, H. J., Patel, R., Im, S. J., Kim, J. H., Min, B. R., 2007. Silver nanoparticles immobilized on thin film composite polyamide membrane: Characterization, nanofiltration, antifouling properties. Polymers for Advanced Technologies 18 (7), 562-568.45 Rana, D., Kim, Y., Matsuura, T., Arafat, H. A., 2011. Development of antifouling thin-film-composite membranes for seawater desalination. Journal of Membrane Science 367 (1-2), 110-118.46 Yin, J., Yang, Y., Hu, Z. Q., Deng, B. L., 2013. Attachment of silver nanoparticles (agnps) onto thin-film composite (tfc) membranes through covalent bonding to reduce membrane biofouling. Journal of Membrane Science 441, 73-82.47 Yang, H. L., Lin, J. C. T., Huang, C., 2009. Application of nanosilver surface modification to ro membrane and spacer for mitigating biofouling in seawater desalination. Water Research 43 (15), 3777-3786.
The potential to incorporate metal nanoparticles, for example silver nanoparticles (Ag-NPs) and copper nanoparticles (Cu-NPs) discussed above, as biocides in membranes for water purification has gained much interest in recent years. However, a viable strategy for loading biocidal metal nanoparticles on water purification membranes remains challenging.